trivial: Allow Application() to have a None package
Split up the webapps into different files
trivial: Add a __str__ function to Application
Do not abort the build if one AppData description is corrupt
Add some screenshots for the web applications

These are not picked up yet, as the builder needs some tweaks to pick them up.
trivial: Add a __str__ function to Screenshot
Use Unicode internally everywhere

The current mix of Unicode and UTF-8 is not good for my mental health.
Build extra appstream files into the project file

This allows us to extract and process the screenshots at the same time.
Remove upstreamed AppData files